The bad news is that, for Rossi at least, even the favoured prototype engine on its own does not offer the step forward the M1 needs to be a firm victory contender.

Speaking after day one of the Jerez test, Rossi said he and Vinales prefer the version with more engine braking: "The way is quite clear for the engine choice. But from what I feel it doesn’t make a lot of difference.

"The problem is that for me we don’t make a big step. If you compare to the other top guys, more-or-less our speed is the same as this year."

Rossi was ranked just 17th fastest during day one at the final MotoGP test of the year, following a technical problem when he went out on a new tyre.

"I feel a strange noise, a bad noise, and I stop. I don’t know what happened."

The bigger issue for the Italian remains race pace, with the M1 still using the rear Michelin tyre less efficiently than its main rivals.

"For me the problem is this: With the new tyre, the bike is very good. Maverick is very fast. Also Franco, who doesn’t have a lot of experience, is always good. But if you check deeply, after four or five laps, for some reason we suffer more.

"So more-or-less, for me, Yamaha have to be clear that [the 2019 engine] is not enough.

"For me, if we race tomorrow we are fifth, sixth, seventh. Maybe fourth if one crashed in front. But we don’t fight for the victory."

The Doctor emphasised: "It's very easy to choose between the engines we have now. The problem is not to make the right choice, the problem is that anyway one or the other is not the improvement we need."

That begged the question of whether Yamaha, which wanted to make a final 2019 engine decision at this week's test, will instead work on a third specification for next year's opening test at Sepang.

"I don’t know sincerely the development plan of Yamaha. If they will do another one, another step, I don’t know," said Rossi.

However the Italian, winless since Assen last year, is still holding out hope that other new parts - such as electronics - can offer a step forward in the areas where the M1 is lacking.

"We don’t have just the [new] engine, we have also some other areas that you can work on and modify, also during the season. So we hope that we can improve."

Vinales was fourth fastest and Morbidelli sixth, as Ducati riders Petrucci and Dovizioso set the Wednesday pace.

"Today the Ducatis are quite impressive. I have to check better the pace, maybe I'm wrong, but the lap time that they do... easy. For me, this is 'boring' for us!"

The second and final day of MotoGP testing takes place at Jerez on Thursday.